With development of communications technologies, an operating frequency band of a communications system is increasingly wide, and the communications system also receives more unwanted signals and various types of noise. Therefore, a modern communications system needs to strongly suppress out-of-band signals, and has a more complex design. A filter having a favorable out-of-band suppression function can greatly simplify complexity of the communications system, simplify a design, and reduce costs. In an existing filter with a wide stopband, a high-order harmonic wave of a baseband signal is suppressed by increasing transmission zeroes in many designs. In addition, in some filters, a wide stopband is generated in a stepped impedance resonator (SIR) manner.
In a prior-art solution, a series of short-circuit lines is added to a quarter-wave coplanar waveguide (CPW) resonator, to generate extra out-of-passband transmission zeroes. These transmission zeroes may be used to suppress harmonic waves, to implement relatively favorable out-of-band suppression. In addition, multiple identical resonators are cascaded in this solution, to enhance an out-of-band harmonic-wave suppression effect. However, a stopband in this solution can be expanded to only five times of a baseband signal. Moreover, a filter area designed in this technical solution is relatively large, and costs are relatively high.
In another solution, a hybrid structure of a microstrip and a CPW is used. The microstrip and the CPW may be mutually converted to generate extra open-circuit lines, to generate transmission zeroes. Likewise, these transmission zeroes can be used to suppress a high-order harmonic wave of a baseband signal. However, in this technical solution, similarly, a stopband can be suppressed to only 6.52 times of a baseband signal, and an area is relatively large. This causes higher costs of an entire filter.